(1) Field of the Invention
This invention relates to magnetic resonance imaging OMRI) apparatus. More particularly, the invention relates to a magnetic resonance imaging apparatus including an RF coil having an approximately cylindrical shape for picking up sectional images of predetermined sites such as the neck, head or knees.
(2) Description of the Related Art
A conventional RF coil for picking up sectional images of the head and neck is constructed as shown in FIGS. 1 and 2, for example. FIG. 1 is a view in vertical section showing a construction of a conventional RF coil and adjacent elements for picking up images of the head and neck. FIG. 2 is a view seen in a direction indicated by arrows 2--2 of FIG. 1.
Apatient or examinee M is placed on a top board 100. The top board 100 is horizontally movably supported on a bed stand (not shown). Sectional images are picked up of the examinee M inserted into an opening of a gantry not shown.
A headrest 101 is mounted on the top board 100 through a headrest support 102 and a shoulder-rest 103. A predetermined space 104 is formed between the top board 100 and headrest 101.
The headrest 101 has slide rails 105 attached thereto, while an RF coil 106 for head and neck imaging has sliders 107. Thus, the RF coil 106 is slidable along the headrest 101.
The RF coil 106 has a shape of elliptical cylinder defining a hollow space 108. Cutouts 109 are formed at a proximal end (righthand end in FIG. 1) of the RF coil 106 for receiving the shoulders of examinee M.
A static magnetic field is formed in the gantry to extend on a horizontal plane. The RF coil 106 has enhanced sensitivity when adjusted to align the axis 110 of hollow space 108 to the direction of the static magnetic field. It has been conventional practice, therefore, to set the headrest 101 horizontally to the top board 100 as shown in FIG. 1, so that the axis 110 may be aligned to the horizontal. The RF coil 106 is slid horizontally along the headrest 101 maintained in such a state.
With the above construction, sectional images are picked up as follows.
First, the examinee M is laid on the top board 100, with his or her shoulders placed on the shoulder-rest 103 and the back of the head on the headrest 101 (or on a cushion 101a thereof). In this state, the RF coil 106 is slid rightward from a position shown in two-dot-and-dash lines in FIG. 1, to insert an imaging site of examinee M into the hollow space 108 of RF coil 106. Then, the top board 100 is moved horizontally to insert the imaging site into the opening of the gantry to pick up sectional images of the imaging site.
The conventional apparatus of the above construction has the following drawbacks.
When picking up sectional images of the neck, as shown in FIG. 1 for example, the RF coil 106 is set as far rightward in FIG. 1 as possible for the cutouts 109 of RF coil 106 to contact the shoulders of examinee M. This step is taken in order to position the imaging site, i.e. the neck KB, inward from the cutouts 109 since sensitivity is low adjacent the cutouts 109 which would result in poor image quality.
It is therefore necessary to fit the shoulders of examinee M into innermost ends of the cutouts 109. Conventionally, the shoulder-rest 103 is used to raise the shoulders of examinee M to the level of the innermost ends of the cutouts 109.
Consequently, during a neck image pickup operation, the examinee M is forced to take a posture arched from the head to the shoulders as shown in FIG. 1. This forced posture is uncomfortable and could be painful to the examinee M.
In such a posture, the nose and chin of the examinee M thrust upward to have only a narrow space 111 from an inner circumferential surface of RF coil 106 extending over the nose and chin of the examinee M. This gives the examinee M a sense of tightness. In the case of a large examinee M, it may be impossible to insert an imaging site (such as the head) into the hollow space 108 of RF coil 106, hence impossible to pick up sectional images.
By enlarging a dimension (cylinder diameter) of RF coil 106, for example, the examinee M may be given increased freedom for taking a more comfortable posture in the hollow space 108. However, an enlargement of RF coil 106 would result in a decrease in the sensitivity of RF coil 106, leading to poor image quality.